For realizing higher density optical discs and a higher accuracy of optical measurement, a short wavelength light source of a small size is necessary. Particularly, a recording and reproduction method using holograms has been drawing attention as a high density optical disc of a next generation since a recording density of around 100 Gbit/inch2 may be achieved. FIG. 10 is a schematic diagram showing a structure of a holographic optical information recording and reproduction device which has been proposed conventionally. The recording and reproduction device uses an optical disc optical system of a shift multiplex recording mode.
A laser beam emitted from a laser light source (not shown) as a coherent light source is split into two. Then, one beam passes through a space light modulator (hereinafter, referred to as “SLM”) 113, and is converged to a hologram recording medium 115 with a Fourier transformation lens 116 to become a signal light 110. The other split beam is converted to have an appropriate beam diameter by a converging lens 117, and is applied as reference light 111 to a position the same as that of the signal light 110 in the hologram recording medium 115. The hologram recording medium 115 has a structure such that a hologram medium such as photopolymer is sealed between two glass substrates. An interference fringe of the signal light 110 and the reference light 111 is recorded. For reproducing the recorded signal, only the reference light 111 is applied to the hologram recording medium 115. Diffracted reproduction light 112 from the hologram passes through a Fourier transformation lens 118, and is received by a CCD element 114.
In the recording and reproduction device using holograms, angular multiplex recoding can be performed since the thickness of the hologram recording medium 115 is as thick as about 1 mm, and the interference fringe is recorded as a thick grating, a so-called Bragg grating. In the recording and reproduction device of FIG. 10, angular multiplex is realized by shifting the position to which a spherical wave reference light 111 is applied, instead of changing an incident angle of the reference light 111 for shift multiplex recording. More specifically, the hologram recording medium 115 is rotated by a small amount within the plane, and a recording position is shifted. The incident angle of the reference light 111 sensed by each part of the hologram medium is changed slightly. In this way, multiplex recording is performed.
When the thickness of the hologram medium is 1 mm, angular selectivity defined by reproduction signal intensity is 0.014 degrees at the full width at half maximum. When NA of the reference light is 0.5, multiplex of the holograms can be realized in intervals of about 20 μm. The recording density is 200 Gbits/inch2, which is 300 GB when it is converted to the capacity of a 12 cm disc.
The Bragg grating has angular selectivity and also has wavelength selectivity. Thus, it becomes necessary to control the wavelength of the light source when recording or reproduction is performed. The wavelength selectivity in the grating for this hologram medium is 0.2 nm.
For realizing such a high density optical information recording and reproduction device as described above, a small and stable laser light source and a recording medium which allows multiplex recording are important.
FIG. 11 is a schematic diagram showing a structure of an SHG blue light source using alight guide QPM-SHG device, which is a typical stable light source (see Japanese Laid-Open Publication No. 2002-204023). As shown in FIG. 11, a DBR semiconductor laser of a wavelength variable type which has a distribution Bragg reflector (hereinafter, referred to as “DBR”) area is used as a semiconductor laser 101. The wavelength variable DBR semiconductor laser 101 is a 100 mW class wavelength variable DBR semiconductor laser of AlGaAs with a wavelength of 850 nm. The wavelength variable DBR semiconductor laser 101 is formed of an active layer area, a phase adjustment area, and a DBR area. By changing electrical currents flowing into the phase adjustment area and the DBR area at the same time, an oscillation wavelength can be changed continuously.
A fundamental wave P1 from the semiconductor laser 101 is transformed to have half the wavelength in an SHG device 102 which can convert the wave length, and is output as higher harmonic wave P2. The semiconductor laser 101 and the SHG device 102 are fixed on a sub-mount 160, and they are incorporated within a package 150. The higher harmonic wave P2 having the wavelength of 425 nm passes through a window 151 of the package 150 and is taken outside.
As described above, in the recording and reproduction device using holograms, the diffraction pattern to be recorded varies depending upon the directions of incidence of the light, and/or wavelengths. If the wavelength of the light for recording and that of the light for reproduction are different, cross talk signal may increase, and/or intensity of the signal light may decrease.
Information of the hologram recording medium 115 is reproduced as Bragg diffraction light from the recorded interference fringe. For reproducing the information of the hologram recording medium 115 with a sufficient amount of light, Bragg conditions have to be satisfied. The incident angle of the reference light beam with respect to the hologram medium and the wavelength of the reference light beam have to be respectively adjusted to optimal values.
For example, when a system in which the thickness of the hologram medium is 1 mm, the wavelength of the light source is 515 nm, and the interval of the interference fringe is 0.5 μm in the system is assumed, a range of permissible values of the Bragg conditions for the wavelength of the reference light beam, which is defined by a value of the wavelength where diffraction efficiency is reduced by half, is 515±0.2 nm.
In the recording and reproduction device using holograms, a hologram is recorded by the interference of the signal light and the reference light. Thus, when a stray light, which is unnecessary reflected light, is generated in the optical system, the stray light generates unnecessary interference fringe, and holograms cannot be recorded or reproduced stably. In such a case, it is possible to provide a precise optical system on a shock resistant table or the like to eliminate such stray light. However, this increases the size of the recording and reproduction device, and a small recording and reproduction device cannot be achieved.